The racemates produced in the chemical synthesis of amino acids are usually separated into the individual enantiomers by enzymatic splitting. This can be carried out e.g. by stereospecific enzymatic splitting of the N-acylated amino acids or by enantioselective enzymatic splitting of the corresponding hydantoins which are monosubstituted in 5-position. In each case, an unsplit enantiomer of the starting material can be recovered and subjected once again, after a racemization process, to enzymatic splitting. A good survey of possible racemizations is provided by E. Adams in P. D. Boyer, "The Enzymes", vol. VI, New York, 1972, pp. 479-507 (Amino Acid Racemases and Epimerases). In principle, the racemization process can be carried out chemically, physico-chemically or enzymatically. Combined methods are also possible, depending on process conditions. An enzymatic hydantoin racemization is described by M. Pietzsch in "Stereochemische Untersuchungen zur enzymatischen Hydrolyse von D,L-5-monosubstituierten Hydantoinen" [Stereochemical Investigations on the Enzymatic Hydrolysis of D,L-5-Monosubstituted Hydantoins], thesis, Technical University of Braunschweig, 1989, in which a racemase from Arthrobacter spec. (DSM 3747) was used. However, enzymatic methods have the disadvantage that the known racemases have high substrate specificity and therefore can only be used for very special methods. For many hydantoins, no racemases are known yet.
For this reason the chemical or physico-chemical racemization predominates in the case of the hydantoins as well as amino acids. In such racemization the educts are usually subjected to a temperature-time program under basic conditions. Methods are also known for amino acids in which the racemization is carried out on strongly basic anionic exchangers; in those cases, relatively low temperatures (25.degree.-45.degree. C.) are sufficient. However, these anion exchanger methods are only successful in the case of neutral or basic amino acids and require the use of copper(II) ions; in those cases, a reactive copper(II) Schiff's base complex is produced as an intermediate. G. C. Barrett, Amino Acids and Peptides 20, pp. 1-51 (1989) provides a survey of these methods, and special methods are described in Makromol. Chem. 187, pp. 1065-1076 (1986). These methods are extremely expensive and can only be carried out on a laboratory scale since, in addition to the copper ions an ion exchanger neutralized with 4-hydroxy-3-formylbenzene-sulfonic acid (5-sulfosalicylaldehyde) is used. This material requires a regeneration. It also requires an expensive purification of the products obtained, since the copper ions must be removed. quantitatively. Moreover, the use of transition-metal cations has the disadvantage that very stable complexes can possibly be formed with the amino acids used.
The chemical racemization of 5-monosubstituted hydantoins runs via keto-enol tautomerism (Chem. Rev. 46, pp. 403-470 (1950)) and is catalyzed, as was determined very early on, by bases (Am. Chem. J. 44, pp. 48-60 (1910). More recent investigations on the chemical racemization of hydantoins were carried out only with special substrates, e.g. 5-(p-hydroxyphenyl)-hydantoin in Agric. Biol. Chem. 51, pp. 721-728 (1987) and 5-benzylhydantoin in J. Org. Chem. 55, pp. 4755-57 (1990). These investigations show that a rapid racemization occurs only in the case of 5-(p-hydroxy-phenyl)-hydantoin because of resonance stabilization by the 5-substituent (50% after 20 min.)--all other hydantoins racemize only after several hours.